As mentioned above a reactor of a nuclear plant comprises a core with a plurality of fuel assemblies. The fuel assemblies are vertically distributed and each fuel assembly contains a plurality of fuel rods. Each fuel rod comprises a cladding which encloses the nuclear fuel in the form of pellets. Common nuclear fuel material is uranium and/or plutonium. During the normal operation of the nuclear plant the nuclear fuel in the fuel rods is burned up, leading to the formation of fission gases that comprise radioactive inert gases. These fission gases normally stay inside the fuel rods.
The environment inside the reactor is demanding for the components positioned therein. The environment is for example highly oxidative and the components are exposed to strong radiation. Furthermore, the generated power inside the reactor is not uniformly distributed throughout the core and some parts are exposed to higher local power levels than other parts. The local power levels may vary for example when a control rod is moved or the water flow and/or the water temperature is changed. The nuclear power producers constantly aim at producing more power, i.e. increasing the effectiveness of the nuclear plant. It is for example desirable to run the fuel assemblies for as long operation cycles as possible to reduce the outage time for refueling. There are however certain limits in the operating conditions of the nuclear plant that are not to be exceeded, to avoid damaging the fuel, and these limits must therefore be carefully monitored.
Sometimes during normal operation of the nuclear plant a defect in the cladding of a fuel rod appears. Such a defect may lead to the release of the above mentioned fission gases produced inside the fuel rod. The defect can be of a primary or a secondary nature. A primary defect is the first defect that appears on the cladding. It can appear due to for example mechanical wear or a local power hot spot and is normally a small hole or crack in the cladding. The primary defect may over time develop into a secondary defect which is a larger hole or crack in the cladding. A secondary defect may lead to serious damage on the cladding and eventually a failure of the fuel rod, which in turn may lead to a release of nuclear fuel material into the reactor water. A single fuel rod failure in a fuel assembly could lead to exceeding the allowed radioactivity levels in the coolant, forcing a shutdown of the nuclear plant. Hence, as mentioned above, it is important to monitor the nuclear plant and to be able to effectively locate fuel assemblies containing defect fuel rods either for their removal or for modifying the plant operation to avoid a secondary defect. A fuel assembly containing a defect fuel rod must be removed in order to prevent a total failure of the reactor.
One way of monitoring the operation of a nuclear reactor is to use a system that detects the release of fission gases from the fuel assemblies. These kinds of systems are sometimes called activity monitoring systems. The release of fission gases is an indication that a defect on a fuel rod has appeared. The nuclear plant can however continue to be run by for example reducing the power in the part of the reactor where the fuel assembly containing the defect fuel rod is positioned. The fuel assembly in question may thereafter be removed when the operation cycle is over and the reactor is shut down to be charged with new nuclear fuel.
To be able to continue running the nuclear plant it is therefore important to find out in what part of the reactor the defect has appeared. A well-known way of doing this is by a method called flux-tilting or power suppression testing, described in U.S. Pat. No. 5,537,450 A. Flux-tilting involves the movement of control rods up and down in the reactor. A control rod is made of a material that is able to absorb neutrons without fissioning itself. The control rods are therefore able to slow down the fission of the nuclear fuel and thereby reduce the power generated in its vicinity. The control rods are distributed throughout the core of the reactor and can be independently moved up and down to control the power in different positions of the core.
In the flux-tilting method the control rods are moved up and down in the core of the reactor and at the same time the off-gas stream from the reactor is analyzed for the detection of fission gases. When a control rod is inserted further into the core the power is reduced. When that control rod is then pulled out from the core the power is increased and more fission gases are produced in the fuel rods leading to a higher release of fission gases through a possible defect. By independently moving control rods at different positions it is in this way possible to locate in what part of the core the defect has appeared.
Flux-tilting is however not free of risks as the method itself may lead to a higher risk of a secondary defect due to local power changes. Therefore flux-tilting should be carried out at reduced reactor power. A reduced reactor power results in a decrease of the effectiveness of the nuclear power plant and, hence, a production loss.
A further way of monitoring the operation of a nuclear plant would be to use the information available from a system that continuously calculates the power distribution in the core of the reactor. The calculations could be made by advanced computer programs that use a number of measured process parameters obtained from the core. Such calculations may result in three dimensional power distribution patterns showing power peaks and power depressions for different positions of the core. The system would render it possible to make comparisons over time in order to observe where changes in the power take place. It is possible to infer the location of a defect on a fuel rod by observing these power changes, but some defects on the fuel rods appear without any prior change in the fuel assembly output power, such as those caused by mechanical wear.